1998 KX125 bogging. Help!!!

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Mar 10, 2004
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#1
Hi, ive got a 1998 KX125 and ever since day one its bogged real bad off the bottom. Ive ridden heaps of my mates 125s and my old 125s and none of them bogg as bad as this. I know 125s have a bogg at the bottom but not this much.
Going from closed throttle to wide open it makes a woooo-oom sound and thats in nutural. And when taking off you have to feather the clutch like mad to get it to go. The strange thing is this bike is super fast when its in power band and is great fun to ride, apart from having to constantly clutch and down shift to keep it reving.

So what have I down about it? I have put in a new plug, new piston, ring and base gasket. Have serviced the powervalves and have had the carb set up properly at the the shop. It is running a very clean twin air filter with stock jetting and stock front bag pipe and rear muffler (freshly packed). It has boyseen carbon fibre reeds which look to be in good condition. I live in New Zealand and we have 98 octane pump gas and I use castrol TTS mixed at 30/1

This bike really has me stumped! What could it be? the timming is spot on and the jetting is stock as is the rest of the bike except the reeds and yet it bogs real bad.
Could the jetting somehow be wrong? or is it more likely a crankcase leak or something I may have over looked.
I welcome any ideas please :bang:
 
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May 1, 2004
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#2
Try

Pulling up the choke a little prior to opening her up to WOT.
From what you say it sounds as if its to lean.
Pulling the choke will richen the mix and if it helps you know your to lean.
Just a thought.
Generally when to rich in going from idle to WOT it will make more of a crackling sound and not your typical Bwoooooooooha.
Also you say from "day one" so its done it since 98???
Did you just buy it recently and has done it ever since you purchased it?
Best of luck.
 
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Aug 21, 2003
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#3
hello i come from nz as well. i use to use tts but stoped because i was told it cloges everything up. i also found i have better response and my bike funs nicer using motul 800. what part of nz u from. im from canty
 
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#4
I will start the bike tomorrow and try the choke thing, It has done it since I purchased it 3-4 months ago. I wasnt able to take it for a ride when I brought it, could only just hear it running.

Im from Palmerston North bro, I know Jeremy McKnight from down those ways.
 
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#5
Ok I tried the choke and it made it worse.
But the more I wind in the air/mixture screw the better it runs. With it wound fully in it runs the best???
Any ideas?
 
- a d v e r t i s e m e n t -

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#6
Well,

By turning in the Airscrew you are enrichening the Idle Circuit but I cant see that making a diffrence beyond 1/8-1/4 throttle.
Beyond a 1/4 throttle the Needle Clip position then comes into play and it is overlapped from 1/2 to Full throttle by your Main Jet.
Each Turn in of the AirScrew = is like going up 1 Slow Jet (Pilot Jet for a 125) at a time.
I would simply pull the carb and inspect, clean it and see just what jets are in there.
You might have a loose jet? Dirt in there?
Atleast if you take it apart and clean it you can then rule those out and concentrate on getting it jetted correctly.
 
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#7
Yo bro. I am also from NZ , Akl. I had kind of the same prob when i first bought my bike used wrong fuel octane level. Try using 91 pump gas. Or use 98 but u have to re-jett.

Hope that helps
 
- a d v e r t i s e m e n t -

CaptainObvious

Formally known as RV6Junkie
Damn Yankees
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Jan 8, 2000
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#8
Rotaholic said:
...the jetting is stock...
And there is your problem. You need to spend a day with some jets and a screw driver. There is a lot of information on jetting here on DRN, so rather than me try to explain it, please do a search. The KDX forums have the best info on this subject.

I will give this advice:start with a clean air filter, fresh spark plug and fresh gas. Make sure there aren't any air leaks in the intake boot and make sure that your power valves are functioning properly.